A wireless communication system typically has mobile terminals 10, wireless base stations 30, mobile communication core network 50, and network managing system 90, as shown in FIG. 1.
Each wireless base station 30 is each provided with wireless cell 35 as a range in which wireless links 20 are effective with mobile terminals 10 in the vicinity of its own station and manages this wireless cell 35.
Mobile terminals 10 connect with wireless cells 35 that are managed by wireless base stations 30 and carry out transmission and reception of the data of communication traffic and control traffic with wireless base stations 30.
Wireless base stations 30 further carry out transmission and reception of data with mobile communication core network 50 that is connected by way of wired links 40.
Mobile communication core network 50 includes an exchange station (not shown) and server device (not shown), is both connected by way of wired links 40 to wireless base stations 30 and connected by way of wired link 60 to outside network 70, and carries out transmission and reception of data with wireless base stations 30 and outside network 70.
Network managing system 90 is both connected to wireless base stations 30 by way of wired links 80 and connected to mobile communication core network 50 by way of wired link 81 and carries out monitoring of the quality of the wireless communication system, fault monitoring, and configuration management.
In a wireless communication system of this type, it is necessary that the wireless parameters of wireless base stations 30 be set appropriately to satisfy a required wireless quality in all service areas. Representative examples of wireless parameters include the total transmission power of each wireless cell, tilt angles in vertical/horizontal planes of wireless base station antennas, neighbor cell lists, and handover thresholds.
In the following description, it is assumed that a neighbor cell list of wireless cells managed by its own station is set as a wireless parameter in each wireless base station 30, and this neighbor cell list is next described.
In a wireless communication system, service is offered by a planar arrangement of wireless cells in all service areas. When moving from the connection destination wireless cell to another wireless cell, mobile terminal 10 carries out a process called handover of switching the connection destination wireless cell. This process is realized as follows. Mobile terminal 10 is first instructed in advance to measure the wireless quality of the connection destination wireless cell and wireless cells that are adjacent to this cell (neighbor cells), and if the wireless quality of the connection destination wireless cell deteriorates, to report the measurement information of the wireless quality of neighbor cells to wireless base station 30 that manages the connection destination wireless cell. Wireless base station 30 then determines the handover destination wireless cell at the point in time when a report is received from mobile terminal 10.
At this time, a method of limiting handover destination wireless cell candidates to only a portion of neighbor cells is typically used to reduce the load of mobile terminal 10 and effect rapid processing of the handover. The neighbor cell list is a list indicating, from among neighbor cells, those wireless cells that are candidates for handover destination of mobile terminal 10. A neighbor cell list is generated for each wireless cell by the communication provider and is communicated to mobile terminals 10 by way of a downlink line from wireless base station 30 that manages that wireless cell. Mobile terminals 10 may measure the wireless quality of, from among neighbor cells, only those wireless cells that are registered in the neighbor cell list and then report to wireless base station 30. In the following explanation, among neighbor cells, the wireless cells that are registered in the neighbor cell list are referred to as “Listed Cells,” and wireless cells that are not registered on the neighbor cell list are referred to as “Detected Cells.”
An example that uses Equation (1) disclosed in Non-Patent Document 1 is here described as an operation condition for mobile terminals 10 to carry out the operation of reporting measurement information of the wireless quality. In this example, mobile terminals 10 measure, as the wireless quality of a wireless cell, the reception power of a pilot signal transmitted from wireless base station 30 that manages the wireless cell.[Equation 1]Ps+Os<Pt+Ot  (1)
In Equation (1), Ps is the reception power in mobile terminal 10 of a pilot signal transmitted from wireless base station 30 that manages the connection destination wireless cell. Pt is the reception power in mobile terminal 10 of a pilot signal transmitted from wireless base station (neighbor base station) 30 that manages a neighbor cell. Os and Ot are both offset values of reception power, Os acting upon the reception power of a pilot signal transmitted from wireless base station 30 that manages the wireless cell of the connection destination, and Ot acting upon reception of a pilot signal transmitted from neighbor base station 30. Different values of Ot can be set for each neighbor cell.
When the operating condition of Equation (1) is set, wireless base station 30 communicates the operating condition of Equation (1) to mobile terminals 10 that are connected to the wireless cell that it manages.
Taking as a trigger the satisfaction of the operating condition of Equation (1) by the reception power of pilot signals transmitted from each of wireless base station 30 that manages the wireless cell of the connection destination and those neighbor base stations 30 that manage Listed Cells that are registered in the neighbor cell list, from among neighbor base stations 30, mobile terminals 10 report to wireless base stations 30 the reception power of pilot signals transmitted from neighbor base stations 30 that manage Listed Cells as the measurement information of wireless quality.
In mobile terminals 10, satisfying of the operating condition of Equation (1) is the trigger for reporting measurement information of wireless quality to wireless base station 30, and Equation (1) is therefore sometimes referred to as the “trigger condition” in the following explanation. Equation (1) may further be rewritten as Equation (2) and parameter ThHo may be referred to as the threshold value for determining handover (handover threshold value). This is because Equation (1) is used as the trigger condition for reporting from mobile terminal 10 to wireless base station 30 that the wireless quality of the wireless cell that is the connection destination has deteriorated when judging whether to execute handover.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                            {                                                                                                  P                    t                                    -                                      P                    s                                                  >                                  Th                  HO                                                                                                                          Th                  HO                                =                                                      O                    s                                    -                                      O                    t                                                                                                          (        2        )            
When the measurement information of wireless quality of the connection destination wireless cell and the Listed Cells satisfies the above-described operating condition, mobile terminals 10 report the measurement information of the wireless quality of these Listed Cells to wireless base station 30.
Wireless base station 30 normally determines the wireless cell of the handover destination from among the Listed Cells for which reports have been received from mobile terminals 10. As a result, mobile terminal 10 is normally not capable of handover to a Detected Cell that is not registered in the neighbor cell list.
Accordingly, when a wireless cell has been omitted from registration in the neighbor cell list, the concern arises that an abnormal disconnection may occur in the middle of communication without the ability to implement handover to an appropriate wireless cell or that wireless quality may deteriorate because handover was implemented to an inappropriate wireless cell. As a result, the generation of a neighbor cell list having no registration omissions is crucial for maintaining good wireless quality.
On the other hand, in order to limit the load on lines required for reporting neighbor cell lists as well as the load on mobile terminals 10 required for the measurement and communication of wireless quality, an upper limit (LMax) is normally placed on the number of wireless cells that are registered in a neighbor cell list. Accordingly, priority must be given to the registration of wireless cells having a high potential of contributing to the improvement of wireless quality in order that such wireless cells be registered in a neighbor cell list.
However, setting a neighbor cell list typically requires advanced technology, and the updating (optimization) of a neighbor cell list is normally carried out based on, for example, the results of a traveling (running) test in the service area of a wireless communication system. However, in recent years, neighbor cell list optimization systems are being investigated that autonomously update neighbor cell lists during operation of a wireless communication system.
FIG. 2 shows an example of the configuration of a wireless communication system that is provided with a neighbor cell list optimization system.
The wireless communication system shown in FIG. 2 includes: mobile terminals 10, wireless base stations 30, mobile communication core network 50, network managing system 90, and neighbor cell list optimization system 95. In FIG. 2, constructions that are the same as FIG. 1 are given the same reference numbers and redundant explanation is here omitted.
Neighbor cell list optimization system 95 is connected to network managing system 90 by way of wired link 82 and carries out updating of the neighbor cell lists of wireless cells 35 that are set to wireless base stations 30.
Non-Patent Document 2 discloses a neighbor cell list update method realized by a neighbor cell list optimization system.
FIG. 3 is a view for describing the updating method of the neighbor cell list described in Non-Patent Document 2. In Non-Patent Document 2, as the wireless quality of a wireless cell, a mobile terminal measures the reception power of a pilot signal that is transmitted from the wireless base station that manages the wireless cell. In addition, a mobile terminal measures both Listed Cells and Detected Cells as neighbor cells, and when the reception power of the pilot signal transmitted from the wireless base station that manages a neighbor cell is at least a threshold value, reports the reception power to the wireless base station as measurement information of the wireless quality of that neighbor cell.
As shown in FIG. 3, in the neighbor cell list update method described in Non-Patent Document 2, the number (a) of trial handovers to Listed Cells that are registered in the neighbor cell list is totalized for each of these Listed Cells. In addition, the report number (n) is totalized for each Detected Cell. The report number (n) is the number of receptions of reports from mobile terminals, in which the report is that the reception power of a pilot signal transmitted from the wireless base station that manages the Detected Cells is equal to or greater than the threshold value. Listed Cells for which the number (a) of trial handovers is no greater than a threshold value (ThDel) are deleted from the neighbor cell list, and Detected Cells for which the number of reports (n) is equal to or greater than a threshold value (ThAdd) are registered in the neighbor cell list in order beginning from wireless cells for which the number of reports (n) is greatest. The neighbor cell list is in this way updated.